I. Field of the Invention
This invention relates to radiant heating control systems by which to melt the thermoplastic matrix of a travelling web of thermoplastic composite tape for application by a tape dispensing apparatus. More particularly, the present invention relates to such a heating control system in which melting of the thermoplastic matrix of the travelling web is reliably accomplished irrespective of the velocity or changes in velocity of the travelling tape web.
II. Description of the Prior Art
By way of background, computerized tape-laying machines have been utilized to apply strips of composite tape material to a layup tool or the like to build up or form a part such as an aircraft wing, for example. One such composite tape material includes graphite fibers and epoxy embedded in a thermosetting plastic matrix. To apply the tape, a computerized tape laying machine may include a tape dispensing apparatus or tape applicator which is positionable and movable under computer control relative to the layup tool to pay-off and apply strips or courses of tape to build up multiple plies of tape forming the desired part. Conventionally, the tape applicator is suspended from a carriage which in turn is suspended from a gantry supported off the ground by pylons. Each of the applicator, carriage and gantry are movable such that tape may be applied to the layup in any desired pattern under control of a computer program. An example of one such system is described in U.S. Pat. No. 4,719,397 assigned to the assignee hereof, the disclosure of which is incorporated herein by reference.
Conventionally, tape dispensing apparatus of the type described in U.S. Pat. No. 4,719,397 have been used to apply thermosetting composite tape as above-described to the mold or layup tool. Thermosetting composite tape is easy to work with in that it is moldable and tacky at or around room temperature. While thermosetting composite tape has many desirable attributes, use of such tape does have some disadvantages. For example, once a part is formed with plies of thermosetting composite tape, the part must be cured. Such a cure process may take a great deal of time and energy. Further, such tape, once cured, may not be reformed. Thermosetting composite tape may also have an undesirably limited storage life.
Thermoplastic composite tapes have been developed which are believed to be superior to thermosetting composite tape. Such tapes utilize a thermoplastic matrix supporting the graphite fibers rather than a thermoset plastic matrix. In particular, certain new resins, particularly polyetheretherketone (PEEK available from ICI Americas, Inc.) and polyphenylene sulfide (PPS, also called RYTON, available from Phillips Petroleum) and the polyimides provide a thermoplastic matrix with properties equivalent to or superior to those of a typical thermoset plastic matrix.
Further, to cure thermoplastic composite tape after it is applied is not generally as energy-intensive and time-consuming as the cure for thermosetting composite tape. Despite superior material properties, however, tape having a thermoplastic matrix is difficult to manipulate to form the parts initially. Unlike thermosetting composite tapes which have natural adhesive properties due to the epoxy in the matrix, thermoplastic composite tape is not sticky (tacky) at ambient temperature, and will not readily adhere to previous plies of such tape or to a mold surface. Additionally, in its natural state, thermoplastic matrix material is very stiff and virtually nonconformable. Thus, to build components from thermoplastic composite tape requires that the tape be heated to at least the melting point of the thermoplastic matrix, such as 450.degree. F. or higher, and usually above 650.degree. F. (for PEEK), for example. The treated tape must then be fused to the previous plies of tape in its molten state, and the combined plies cooled under pressure to avoid separation.
Infrared or quartz bulb heaters have been proposed for heating the thermoplastic composite tape. However, control of the heaters so as to reliably melt the thermoplastic matrix is difficult in computerized tape laying apparatus. As is typical with such tape applicator systems, the various movements of the system result in widely fluctuating velocities of the tape as it is applied to the surface. For example, the tape may be moving at a velocity between zero and twenty inch/second and with possibly dramatic changes in velocity. Thus, if the heater bulbs are energized to emit radiant energy at a level sufficient to melt the thermoplastic matrix when the tape is moving relatively slowly, such as at 1 inch/second, the thermoplastic matrix will not be melted when the tape is moving relatively rapidly. Moreover, unless the energy level for the heater is raised accordingly, the tape will not be melted as it accelerates. Similarly, if the heater bulbs are energized to emit radiant energy at the higher level necessary to melt the thermoplastic matrix during periods of high speed travel of the tape, the tape will likely be burned or otherwise damaged as the tape velocity is reduced. Thus, heating of the tape must be carefully regulated so that melting is achieved without burning or otherwise damaging the tape. It is believed, however, that merely varying heater energization level proportionally with the velocity of the tape is insufficient to reliably melt the thermoplastic matrix over the range of speed and acceleration encountered in typical computerized tape laying applications.
The task of reliably melting the thermoplastic matrix of the tape is further complicated because the mechanics of a computerized tape laying system minimize the area available to devote to heating the tape. That is, the temperature of the moving tape must be elevated as much as about 600.degree. F. above room temperature at a point just prior to application so that it does not appreciably cool before contact with the surface. In a computerized tape laying machine, therefore, heating must take place on the movable tape applicator leaving very little space for the heater. As a consequence, the tape may be exposed to the heat source for only a very brief duration during which the temperature of the tape must be rapidly elevated.